I have IBS-D, so I was very reluctant to even take fiber. It took four people, including one doctor, to convince me to try it. I'm glad I did because it helps me so much. For people with IBS-C, it helps with their constipation; makes them more "regular." For people like me with IBS-D, it helps to form bulk making the stools more formed rather than loose. Why can't you take fiber? If you find it to be too rough, it would be beneficial to try fiber that does NOT contain Psyllium. That's why I take Citrucel. It is the least rough, yet most effective fiber for me. The helpforibs site I mentioned to you also has a "Tummy fiber."
I know when you are trying things to help with your IBS it can be like trial and error, which is so difficult to have to deal with. That's why this site is so good for sharing information.
I just realized that I didn't answer your question about the peppermint. What it does for me is reduce the gas, bloating, and cramping. Believe me, I still get that; but not as bad with the peppermint. I also make a point of using the bathroom soon after every meal whether I feel like I need to go or not. That helps me to feel better throughout the day.